First of all, they haven't accomplished much so
far.

These attacks do make Kim Jong-un's regime look silly to the
outside world, but given that he looks pretty ridiculous
already, that's an extremely minor victory. Internally, most
North Koreans will never know about these attacks as they cannot
access the Internet.

Anonymous claims to have gained access to North Korea's real
online network — the walled-off internal intranet and mail
servers known as "Kwangmyong" — but these claims seem unlikely.

The same message speaks to both North Korean leader Kim Jong-un
("First we gonna wipe your data, then we gonna wipe your badass
dictatorship 'government.'") and the people of North Korea ("To
the citizens of North Korea we suggest to rise up and bring these
motherfuckers of a oppressive government down!").

Unfortunately, very few North Korean citizens
will ever even read the message, given that they cannot access
the Internet. Even if they did, rising up to "bring these
motherfuckers of a oppressive government down" is harder said
than done.

The hack and the poster will likely be
deeply embarrassing for the people running
Uriminzokkiri and could land them in serious trouble with the
authorities in Pyongyang.

NK Tech points to reports after a 2011 attack that says that
operators of Uriminzokkiri, based in the Chinese city of
Shenyang, were "questioned" over
their response to the hack. Given that Anonymous has
published email addresses of those hacked, it seems fair to
assume these people will face something similar.